There is known a method for simultaneous determination of the content of several chemical elements, such as nitrogen, phosphorus and potassium, in plant samples. According to this method, samples to be analyzed and standard samples are exposed to a fast neutron flux. After a certain period of time, the spectrum of the gamma radiation induced in the samples is recorded and analyzed with reference to the spectra of the standard samples by using the least squares method (cf. R. A. Srapenyants, J. L. Kovtoun, E. Vernin, G. Aude, C. Axelrod, "Methode et installation de dosage automatique par activation neutronique de N, P, K, Ca dans les vegetaux", Proceedings of the Symposium on Nuclear Activation Techniques in the Life Sciences, Apr. 10-14, l972, Bled, Jugoslavia, International Atomic Energy Agency, Vienna, 1972).
The method under review is disadvantageous because of its low accuracy, especially when analyzing plant samples with a silicon content of more than 0.05 percent by weight. This is due to the fact that the phosphorus content is determined from the radioactive isotope Al.sup.28 which is also produced from silicon as a result of a competitive nuclear reaction. In the case of phosphorus determination, the time between the end of irradiation and the moment the spectrum is taken is such that the contributions of phosphorus and silicon to the spectrum of the irradiated plant sample practically cannot be differentiated.